Add indexes to {metatags} table

The metatag table already has indexes. So there is little extra which can be done to improve a query with a long list of nodes to be loaded, such as the one mentioned at #2139925: Very slow query (using IN operator).

Correction, there is just one primary index for the composite key. Me and @q0rban played with indexes to see if we could optimize the query at metatag_metatags_load_multiple(), until we realized that beta9 already did it by removing revision_ids from the WHERE clause.

I do not know which API functions would benefit from extra indexes at the moment.

I found that adding an index for (entity_type,revision_id) sped up some of the metatag queries significantly. In particular, this query was slow:

SELECT m.entity_id AS entity_id, m.revision_id AS revision_id, m.language AS language, m.data AS data 
FROM metatag m 
WHERE (m.entity_type = 'node') AND (m.revision_id IN (1,2,3,4)) 
ORDER BY entity_id ASC, revision_id ASC;

From the EXPLAIN statement, it showed that in my case, 7500 rows were being scanned. After adding the index, only 4 rows were scanned.

Attached is a patch.